The present invention relates to an OIC/OS material, and especially relates to an OIC/OS material having a stable cubic crystal structure.
Solid electrolytes based on zirconia (ZrO2), thoria (ThO2) and ceria (CeO2) doped with lower valent ions have been extensively studied. The introduction of lower valent ions, such as rare earths (Y, La, Nd, Dy, etc.) and alkaline earths (Sr, Ca and Mg), results in the formation of oxygen vacancies in order to preserve electrical neutrality and this in turn gives rise to oxygen ionic conductivity at high temperature (e.g. greater than 800xc2x0 C.). Typical commercial or potential applications for these solid electrolytes includes their use in solid oxide fuel cells (SOFC) for energy conversion, electrochemical oxygen sensors, oxygen ion pumps, structural ceramics of high toughness, heating elements, electrochemical reactors, steam electrolysis cells, electrochromic materials, magnetohydrodynamic (MHD) generators, hydrogen sensors, catalysts for methanol decomposition, potential hosts for immobilizing nuclear waste and oxygen storage materials in three-way-conversion (TWC) catalysts.
Stabilized ZrO2 has been studied as the most popular solid electrolyte. In the case of doped ZrO2 both partially and fully stabilized ZrO2 have been used in electrolyte applications. Partially stabilized ZrO2 consists of tetragonal and cubic phases while the fully stabilized form exists in the cubic fluorite structure. Both CeO2 and ThO2 solid electrolytes exist in the cubic crystal structure in both doped and undoped forms. The amount of dopant required to filly stabilize the cubic structure for ZrO2 varies with dopant type. For Ca it is in the range of 12-13 mole %, for Y2O3 and Sc2O3 it is greater than 18 mole % Y or Sc and for other rare earths (Yb2O3, Dy2O3, Gd2O3, Nd2O3 and Sm2O3) in the range of 16-24 mole % of Yb, Dy, Gd, Nd, and Sm.
Fully or partially stabilized ZrO2, as well as other commonly studied solid electrolytes, have a number of drawbacks. In order to achieve sufficiently high conductivity and to minimize electrode polarization the operating temperatures have to be very high, in excess of 800-1,000xc2x0 C. For solid oxide fuel cells for example, reducing the operating temperatures below 800xc2x0 C. would result in numerous advantages such as greater flexibility in electrode selection, reduced maintenance costs, reduction in the heat insulating parts needed to maintain the higher temperatures and reductions in carbonaceous deposits (soot) that foul the operation of the fuel cell.
Further, in the automotive industry there is great interest in developing lower temperature and faster response oxygen sensors to control the air to fuel ratio (A/F) in the automotive exhaust. In the case of three-way-conversion (TWC) catalysts solid solutions containing both ZrO2 and CeO2 are used as oxygen storage (OS) materials and are found to be more effective that pure CeO2 both for higher OS capacity and in having faster response characteristics to A/F transients.
Oxygen storage capacities (OSC) in these applications arises due to the facile nature of Ce4+⇄Ce3+ oxidation-reduction in typical exhaust gas mixtures. The reduction of the CeO2 to Ce2O3 provides extra oxygen for the oxidation of hydrocarbons (HCs) and CO under fuel rich conditions when not enough oxygen is available in the exhaust gas for complete conversion to carbon dioxide (CO2) and water (H2O). The use of binary CeO2/ZrO2 and ternary CeO2/ZrO2/M2O3 based catalysts in such applications have advantages over the use of pure CeO2 containing catalysts. This arises because in pure CeO2 only surface Ce4+ ions can be reduced in the exhaust at typical catalyst operating temperatures of 300-600xc2x0 C. (See FIG. 1). However, in binary CeO2/ZrO2 or ternary CeO2/ZrO2/MxOy solid solutions more oxygen is made available through the reduction of bulk Ce4+ and the subsequent migration of xe2x80x98Oxe2x80x99 to the surface of the solid solution crystallites where it reacts with the HCs and CO as is demonstrated in FIG. 2.
The xe2x80x98Oxe2x80x99 migration to the surface of the solid solution crystallites is made possible by the formation of the solid solution and is thus an analogous process to that occurring when these same materials are used as solid solution electrolytes. Thus, a more accurate description of these materials for TWC catalyst applications is to view them as oxygen ion conducting/oxygen storage (OIC/OS) materials. These materials have a much higher oxygen storage capacity compared to pure CeO2, especially after catalyst aging and the formation of large crystallites. Further, the response of these solid solutions to changes in the exhaust gas enviromnent is more rapid compared to pure CeO2 with the result that they operate more effectively in preventing CO/HC breakthrough during accelerations and they further provide oxygen at lower temperatures.
Aging of electrolytes is a phenomena usually associated with a decrease in the ionic conductivity at a constant temperature with time. The aging process is a function of composition, operating temperature, time and temperature cycling. The two main causes of aging are: a) ordering of the cation and anion sublattice and b) decomposition of the metastable phases. In single phase cubic systems the major cause of aging is formation and growth of microdomains and disproportionation at high temperatures into different phases. Aging of cubic Y stabilized ZrO2 oxygen ion conducting electrolytes for example can occur through disproportionation into a Y-rich cubic phase and a Y-poor tetragonal phase. Thus, phase stability at high temperatures is an important property of solid solution electrolytes and maintaining phase stability in an optimized cubic or tetragonal phase after high temperature operation or cycling is a highly desirable property.
For TWC catalyst applications the newest OIC/OS materials consist of a range of CeO2/ZrO2 solid solutions with lower valent dopants added to increase the number of oxygen vacancies and to increase the thermal stability and oxygen ion conductivity of the solid solutions after sintering at high temperatures. Zr-rich compositions have the advantage in that the reduction energies for Ce4+xe2x86x92Ce3+ decrease with increasing Zr content and that the activation energies for mobility of xe2x80x98Oxe2x80x99 within the lattice decreases. This is demonstrated in FIGS. 3 and 4 (Balducci et al., J. Phys. Chem. B., Vol. 101, No 10, p.1750, 1997). However, the Zr-rich systems suffer from the disadvantage in that the OSC capacity is decreased due to the lower CeO2 content. Thus, strategies to optimize the availability (OIC) of the OSC function go counter to those that maximize oxygen storage capacity (OSC).
A further disadvantage of the Zr-rich systems is that the stable crystal structure is tetragonal rather than the more desirable cubic structure. The crossover composition between cubic and tetragonal occurs in the range of 35-45 Mole % ZrO2. Compositions having higher ZrO2 content have the tetragonal crystal structure while compositions of lower Zr content are cubic. Further, composites that increase the facile nature of both the CeO2 reduction and mobility of xe2x80x98Oxe2x80x99 (OIC) within the solid solution lattice at a given and high Ce content are advantageous. This is true not only for oxygen storage (OSC) applications in TWC catalysts but also for preparing highly effective electrolytes in solid oxide fuel cells (SOFCs) where high conductivity at low temperatures is a major requirement.
What is needed in the art are OIC/OS materials having stable cubic crystal structures, and high oxygen storage and oxygen ion conductivity properties.
The present invention comprises an OIC/OS material, a catalyst comprising the OIC/OS material, and a method for converting nitrogen oxides using the catalyst. This OIC/OS material comprises: up to about 95 mole percent (mole %) zirconium; up to about 40 mole % cerium; and up to about 10 mole % yttrium. The invention further comprises the reaction product of up to about 95 mole percent (mole %) zirconium; up to about 40 mole % cerium; and up to about 10 mole % yttrium.
The catalyst comprises: an OIC/OS material having up to about 95 mole percent (mole %) zirconium, up to about 40 mole % cerium, and up to about 10 mole % yttrium; a noble metal catalyst; and a porous support wherein said zirconium, cerium, yttrium, noble metal and porous support are disposed on a substrate.
The method for converting nitrogen oxides in an exhaust stream, comprising: using a catalyst comprising an OIC/OS material having up to about 95 mole % zirconium, up to about 40 mole % cerium, and up to about 10 mole % yttrium, a noble metal catalyst, and a porous support, disposed on a substrate; exposing the exhaust stream to the catalyst; and converting nitrogen oxides in the exhaust stream to nitrogen.
The above described and other features of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.